Red Sea Trough/cyclone development — Numerical investigation

Summary A case of development of a meridionally oriented Red Sea Trough (RST) system and its intensification over the Eastern Mediterranean (EM) region during the ALPEX1982 3–5 March period, is investigated. The MM4 mesoscale model of Penn State University/National Center for Atmospheric Research was first applied for a large scale investigation of the processes. The relative roles of the different acting factors, i.e., terrain, latent heat release and the surface fluxes were calculated employing the factor separation method. Topography and sensible heat flux were found to be the dominant ones.The high resolution non-hydrostatic RAMS 3a model of Colorado State University with nested grids of 100 and 20 km illustrated the finer details of the cyclogenetic processes in the mountainous area of the Abyssinean Highlands, Ethiopia, and the Arabian peninsula, where initiation of the trough took place.Results of the factor separation showed that the topography blocking acted as a cyclolytic factor, preventing the process of the northward trough propagation. The situation changed only after about 30 h of the simulation, when the trough already propagated into the EM area after intensification of the mid-tropospheric westerlies over the central part of the Red Sea area. Starting from this time, terrain was acting as one of two major cyclogenetic factors. The second local effect also working as a cyclogenetic one was the sensible heat flux. Its role was especially important after 36 h of the simulations when strong winds over the sea area caused more active heat transfer from the sea surface to the atmosphere.With 7 Figures